South Carolina Senator Jim DeMint, an ardent voice for conservative principles, announced Thursday that he is resigning from Congress to become president of the Heritage Foundation, a top conservative think tank in Washington.

Wesley Denton, DeMint’s communications director, told TIME the Senator made the decision to leave the Senate Wednesday, and would be addressing Heritage staffers today. He will leave the Senate in early January. Nikki Haley, South Carolina’s Republican governor, will appoint DeMint’s successor; a special election for the seat will be held in 2014. At least two members of South Carolina’s congressional delegation, Republicans Tim Scott and Mick Mulvaney, are thought to be eying a bid for higher office. The state’s senior Senator, Republican Lindsey Graham, is also up for re-election in 2014.

DeMint’s decision, which was first reported by the Wall Street Journal, was a surprise. The South Carolinian coasted to a second term in 2010, and while he had said he didn’t intend to stand for re-election again, his abrupt retirement sent shock waves around Capitol Hill and stripped the Senate of one of its most powerful Tea Party members. DeMint quickly emerged as a general in the conservative insurgency that rose up during Barack Obama’s first term. He became a hero to anti-tax conservatives, crusading against earmarks, helping to foment the Tea Party rebellion and bucking Republican leaders who wavered from the party’s small-government principles. DeMint also led efforts to recruit staunch conservatives to the upper chamber, using his political action committee, the Senate Conservatives Fund, to bolster the candidacies of such new Senators as Marco Rubio and Pat Toomey.

He became the Senate’s conservative scourge, prompting many Republicans to warn that his hard-right ideology and uncompromising approach could alienate many voters and consign the party to perpetual minority status. DeMint was undeterred by such criticisms.”I’m blasting rock, and it’s hard to be graceful,” he told TIME in 2010.

Now he will play the same role a few blocks away from the Capitol, with a bigger salary and perhaps even more clout. “I’m leaving the Senate now, but I’m not leaving the fight,” DeMint said in a statement released Thursday morning. Warren Tompkins, a veteran Republican strategist in South Carolina, said DeMint, “who was basically anti-leadership in a lot of instances,” had a chances to play a more influential role as a conservative enforcer from outside the chamber. “I doubt seriously there’s another elected office on the horizon for him,” Tompkins said. “I think this is something he’s been thinking about and has wanted to do for a long time.”

The Heritage Foundation, where DeMint will replace outgoing President Ed Feulner, is one of the largest and most influential conservative think tanks in the country, and part of its role is holding elected Republicans’ feet to the fire. Earlier this week, Heritage blasted House Speaker John Boehner’s proposal to avert the fiscal cliff, arguing that Boehner had “caved” on raising taxes. Never mind that Boehner had held firm against Obama’s insistence that tax rates rise for the wealthiest Americans — or that it was far more favorable to the GOP than any deal that is likely to be cut. For Heritage, as for DeMint, principle is more important than compromise.